Nuclear fuel for Pressurized Water Reactors (PWRs) is fabricated into fuel assemblies about 12 inches square and 12 feet long. Fissile material in the form of uranium dioxide (UO.sub.2) pellets are stacked within long tubular fuel rods. The fuel rods are themselves arranged in bundles called nuclear fuel assemblies in a square spacing lattice. The fuel rod bundles are in the reactor, forming the reactor core. A few of the fuel rods are left as empty tubes, so that nuclear fuel assembly inserts can be placed within the fuel assembly. Different types of nuclear fuel assembly inserts include Rod Cluster Control Assemblies (RCCAs), Burnable Poison Rod Assemblies (BPRAs), and thimble plugs. The RCCAs, also known as control rods, control the reactor power level. The BPRAs, also known as burnable absorber rods, deplete over an operating cycle to improve fuel utilization. The thimble plugs block empty tubes to improve coolant flow patterns.
The inserts are geometrically similar in their upper portion, but differ in material composition and length. The upper portion is in the form of a central hub sometimes made of radiating spokes. The insert pins, also known as rodlets, are suspended from the hub so as to align the rodlets with the empty tubes in the fuel assembly. The lower ends of the rodlets are unattached and thus freely suspended. The RCCAs and BPRAs are approximately the length of the fuel assembly, while thimble plugs are typically a fraction of the fuel assembly length. A typical fuel assembly will be used for three (3) fuel cycles while many inserts are used for only one cycle (18 months).
After irradiation and exposure to elevated temperatures during the cycle the rodlets can become bowed. Reinserting a fuel assembly insert with bowed rods within a fuel assembly becomes difficult, if not impossible. Also, the unattached lower ends of the rodlets can become entangled and misaligned if released. In reinserting a nuclear fuel assembly insert within the fuel assembly, it is important that the individual rods not become tightly wedged against the guide tubes within the assembly which would make subsequent removal difficult, if not impossible. Moreover, after removal from the nuclear fuel assembly, the assembly inserts are typically stored in a spent fuel pool within spent nuclear fuel assemblies. In order to minimize the volume occupied by each nuclear fuel assembly insert and to accommodate a larger number of spent nuclear fuel assemblies, the inserts must be reinserted and stored in the spent fuel assemblies.
The present invention addresses the aforementioned problems encountered in the prior art by providing a nuclear fuel assembly insert alignment tool which may be manually operated by a single worker for aligning the lower ends of a plurality of fuel assembly insert rodlets and securely maintaining the rodlets in a closely spaced, matrix array for either storage or insertion in a fuel assembly.